KR20030020475A - Process for preparing of a proton-conducting polyvinylidene fluoride membrane - Google Patents

Abstract

PURPOSE: A process for preparing a nanoporous polyvinylidene fluoride membrane (PVDF) having hydrogen ion conductivity and hydrophilicity for use in filtration and electrolysis is provided, in which PVDF membrane with pore sizes mostly of 1 to 100 nm makes hydrogen ion to easily pass the membrane, so that it can be used as hydrogen ion conductor or membrane for water treatment. CONSTITUTION: The process for preparing a nanoporous polyvinylidene fluoride membrane (PVDF) includes the steps of mixing (a) 100 parts by weight of polyvinylidene fluorine resin, (b) 15 to 120 parts by weight of organic acids selected from citric acid, aconitic acid, itaconic acid and their mixtures, and (c) a solvent selected from dimethyl formamide, dimethyl sulfoxide, dimethyl acetamide, N-methyl-pyridone and their mixtures, wherein the solvent is soluble to both the polyvinylidene fluorine resin and the organic acid to prepare a mixture solution; evaporating the solvent from the mixture solution; thermal-forming a resultant solute to modify its surface to be hydrophilic via polymerization of a portion of the organic acid; dipping a resultant material in water or sulfuric acid solution to remove residual organic acid, thus forming nanopores.

Description

Process for producing hydrogen ion conductive fluororesin membrane {PROCESS FOR PREPARING OF A PROTON-CONDUCTING POLYVINYLIDENE FLUORIDE MEMBRANE}

The present invention relates to a method for producing porous hydrogen ion conductive polyvinylidene fluoride (PVDF) resin membranes that can be used for filtration and electrolysis.

PVDF resins are widely used in the environment or metal industry because of their excellent acid and alkali resistance. In particular, PVDF resin is suitable as a filtration membrane that requires acid resistance, and in order to prepare such a filtration membrane, numerous pores must be formed therein.

Membranes are vital for the environment and energy industries, especially hydrogen ion conductors, which are key materials for membrane fuel cells. Hydrogen-conducting membranes basically contain cation-selective groups, Nafion ^R being the flagship product and many other similar products. However, when these conventional hydrogen ion conductive membranes are used in methanol fuel cells, the permeability of methanol is high and the expansion ratio is large, which is unsuitable.

Accordingly, in recent years, a method of making a microporous membrane and inserting an electrolyte solution, particularly sulfuric acid, into pores has emerged. These microporous membranes have advantages of low methanol permeability, low performance deterioration due to contamination of other metal ions, and low dimensional stability of methanol and water.

Before looking at how to make a porous membrane, the materials used to make the membrane, polyvinylidene fluoride (Polyvilylidene fluoride) is excellent in chemical resistance, so much research and development has been done to make the membrane to date. Here, the method of using an inorganic filler that can be dissolved in water to make a porous membrane has the advantage that the manufacturing process is simple, not difficult, and high reproducibility. As inorganic fillers, those using silica, titania, and alumina powder are already known. However, these oxides have a problem that it is difficult to elute them with chemicals because they are very chemically stable.

Gelman's Millipore product is prepared by mixing with other insoluble solvents and causing phase separation through aggregation, which can produce fine pores of 1 μm or less. The pore size of commercialized membranes is 0.005-20 μm.

The method described in U.S. Patent No. 3,518,332 is obtained by mixing PVDF particles and water-soluble metal salt particles in paraffin, mixing them well in the molten state of the resin, and molding them, removing paraffin with an organic solvent and removing metal salts from water. To create a curtain. In this case, the metal salt used is Na ₂ CO ₃ or calcium formate (Ca (COOH) ₂ ).

U.S. Patent No. 4,810,384 discloses a method of mixing PVDF with a hydrophilic resin mixed with water, LiCl, and dimethylformamide and then applying it onto a net, then passing it through a water bath to cause aggregation. That is, a water-soluble metal salt is mixed with PVDF resin to be molded at high temperature, and then eluted in water or an acid solution to make a porous membrane.

However, in the case of using the inorganic filler as described above, it is difficult to disperse and when the polymer film is made, the viscosity increases greatly during molding, making it difficult to make it thin, and there is also a disadvantage in that a large amount of the inorganic filler cannot be added.

Accordingly, methods have been attempted to cause phase separation using organic solvents. Until now, the main methods are to mix the other insoluble solvents and PVDF resins, and then to form and separate the liquid phase to create porosity and to cause gelation.

For example, U.S. Patent No. 3,642,668 discloses a method for casting porosity by casting on a substrate using dimethyl sulfoxide (DMS) and dimethylacetamide (DMAc) as a solvent and then rapidly dipping in methanol to make porosity.

In the method of Japanese Patent No. 51-8268, cyclohexane is used as a solvent, heated, cooled, and cast.

EP 223,709 uses acetone and dimethylformamide (DMF), which rapidly cools the solution and then adds it to a different solvent to create porosity.

In the method described in US Pat. No. 4,203,847, the PVDF resin is dissolved in a hot acetone solution and then poured onto a moving belt, which is immersed in a solvent and nonsolvent solution to form a very thin membrane.

U.S. Patent 4,399,035 discloses dissolving PVDF resin using DMAc and N-methyl-2-pyrrolidone, or tetramethylurea, and a small amount of surfactant (polyethylene glycol, polypropylene glycol, glycerin, or fatty acid esters). Describes how to soak in water or alcohol.

U.S. Patent No. 4,666,607 discloses a thermal gelation process. Here, a U-shaped quenching device is used. The quenching is performed by maintaining PVDF resin mixed with solvent and non-solvent at a temperature above which phase separation occurs.

In EP 378,441 A2, high boiling point diethyl phthalate, dibutyl phthalate and phosphoric acid are added as an organic solvent, and hydrophobic silica powder is added as a filler and then immersed in a 1,1,1-trichloroethane solution after molding. Solvents are eluted and then silica is eluted in a caustic soda solution.

Sami Hitala et al. Irradiated a film made of PVDF resin with an electron beam of 100 k㏉ and then immersed in a styrene solution to produce a polymer of PVDF and polystyrene, followed by sulfonation to provide ion selectivity and hydrophilicity. It was announced.

When making PVDF membranes, such as alcohol fuel cells, hydrophobicity should be eliminated and hydrophilic in order to be used in aqueous solutions. U.S. Patent No. 5,032,331 (Jul. 16, 1991) reports a method of imparting hydrophilicity by immersing the membrane in a strong base solution and removing fluorine ions from its surface.

In addition, as a method for imparting hydrophilicity to the PVDF membrane, it has been studied that the PVDF resin is mixed with a hydrophilic polymer and then molded. U.S. Patent No. 5,066,401 provides hydrophilicity by mixing 2-30% of polymethyl or polyethyl acrylate with 70-98% of PVDF, and the pores present therein have a size of 0.005-10 µm.

As mentioned above, the methods for making microporous, also known as nanoporous, membranes that have been used to date can be broadly classified into two types: porosity by removing an insoluble solvent from a resin, and present in a resin. It is to add a heterogeneous polymer which is easy to leach while being present. The former has the disadvantage that it is difficult to control the process of phase separation and the droplets grow easily to make a small nano-sized space, while the latter is relatively easy to control the pore size and very finely. The advantage is that you can. On the other hand, in the latter method, no organic acid is used as a main component in place of the polymer until now, since most of the organic acid is melted or volatilized at low temperature or adversely affects the reaction.

It is an object of the present invention to basically have a lot of pores of 1 to 100 nm size in the PVDF membrane, and the pores are percolated to allow the electrolyte to enter and exit, thereby allowing hydrogen ions to pass easily, thereby allowing hydrogen ions to pass through. It is an object of the present invention to provide a method for producing a hydrogen ion conductive polyvinylidene fluorine resin membrane that can be used as a conductor or a filtration membrane for water treatment.

Method for producing a hydrogen ion conductive polyvinylidene fluorine resin membrane of the present invention for achieving the above object,

Adding an organic acid and a solvent capable of dissolving both to polyvinylidene fluorine resin to form a mixed solution;

Evaporating and hot forming the solvent in the solution such that a portion of the organic acid is polymerized to form a hydrophilic surface; And

By immersing the molded sample in an aqueous solution of water or acid, the remaining organic acid is removed from the sample to form fine pores so that the electrolyte solution is filled in the fine pores.

Here, the organic acid may be citric acid, aconic acid, itaconic acid, or a mixture thereof, and the organic acid may be added to the polyvinylidene fluorine resin in a ratio of 15 to 120 parts by weight based on 100 parts by weight of solids. desirable.

The solvent capable of dissolving both the polyvinylidene fluorine resin and the organic acid is preferably dimethylformamide, dimethyl sulfoxide, dimethylacetamide, N-methyl-2-pyrrolidone, isophorone, or a mixture thereof.

In addition, the molded sample is preferably immersed in boiling water or sulfuric acid aqueous solution to remove the remaining organic acid from the sample to form fine pores, wherein the aqueous sulfuric acid solution is more preferably 4.9 to 98%.

In the present invention, in order to make a large number of nano-sized pores interpenetrating into the PVDF resin membrane, it is characterized by using an organic acid that is plastic and hydrophilic, and well soluble in PVDF resin. The important thing when making a nanoporous membrane is that the PVDF resin must be mixed very homogeneously, and the organic acid must be effectively leached when leaching into water after molding. The former is made possible by selecting the appropriate additives and solvents, and the latter by selecting the appropriate leaching agent. According to the present invention, the additive is an organic acid which can be chemically uniformly mixed but is not easily separated from citric acid, aconitic acid in which one water molecule is removed from citric acid, and aconic acid. Itaconic acid from which carbon dioxide has been removed can be used as a plasticizer in PVDF resin and can be added to PVDF resin in large quantities. When these organic acids and PVDF resin are mixed and molded, the organic acids are removed from the resin by leaching in water or a suitable solvent or acid to form micro pores in the place where the organic acid escapes. This highly microporous PVDF membrane is formed. In addition, these organic acids remain in the membrane while partially polymerized during molding, thereby imparting hydrophilicity to the PVDF resin membrane.

Hereinafter, the manufacturing method of the hydrogen ion conductive polyvinylidene fluorine resin membrane of this invention is demonstrated concretely.

Citric acid (H ₂ C (COOH) (OH) C (COOH) H ₂ C (COOH)) is an organic acid that can be used at relatively high temperatures, and it can be used as a hydrate of C ₆ H ₈ H ₇ · H ₂ O below 100 ° C. Present and at 100-175 ° C. as citric anhydride. Anhydrous citric acid is melted at 153 ° C, and at 175 ° C it becomes aconitic acid (proponi-1,2,3-tricarboxylic acid, H (COOH) C: C (COOH) CH ₂ (COOH)). . Aconic acid is melted at 191 ° C. and then chemically decomposed into itaconic acid (methylene succinic acid, CH ₂ : C (COOH) CH ₂ COOH). Itaconic anhydride has a melting point of 167 ° C, and the most excellent is aconic acid in terms of heat resistance. When using aconic acid, it is possible to raise temperature to 190 degreeC. If citric acid anhydride is used, the molding should be in the range from 100 to 175 ° C. It is important to note that these three organic acids can be converted and coexist with water as the dehydration and absorption of water occurs depending on the heating conditions. Therefore, it is necessary to pay attention to under what conditions moisture evaporates and leaves no bubbles.

These organic acids are very soluble in water and soluble in methyl or ethyl alcohol, but PVDF resins are insoluble in water or these lower alcohols. Accordingly, suitable solvents capable of dissolving both organic acids and PVDF resins include dimethyl formamide (DMF), dimethyl sulfoxide (DMS), dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone, and isophorone; 3,5,5-trimethyl-2-cyclohexene-1-one). These solvents may be used alone or in combination.

When preparing the hydrogen ion conductive PVDF resin membrane of the present invention, a solution very clear to the naked eye can be obtained by first dissolving these organic acids in the solvent capable of dissolving the PVDF resin and the organic acid together and then adding the PVDF resin. At this time, it is preferable to mix and add an organic acid in PVDF resin in the ratio of 15-120 weight part with respect to 100 weight part of solid content.

The resin solution thus obtained is treated at low temperature to remove the solvent by volatilization. The viscosity of the solution increases during solvent removal, but various molding methods can be used depending on the viscosity. First, in a condition where the viscosity is very low, including a large amount of solvent, the solution is poured onto a metal plate or a glass plate, and then the solvent is evaporated to obtain a clean film free of cracks. There is no crack even when thick films are obtained, because of the flexibility these organic acids give. After the solvent is partially volatilized, it can be drawn into a fibrous condition at relatively high viscosity conditions, and at high viscosity conditions after a large amount of solvent is removed, extrusion, pressing, calendering, and drawing are performed. Molding is possible by the method.

After molding as above, it is immersed in water, alcohol or acid to leach organic acid from the sample. Heating is also a good way to speed up the leaching of organic acids. In addition, immersion in an acid or alkaline solution can increase the leaching rate. In the process according to the invention, experiments showed that concentrated sulfuric acid was the most effective leach agent. At this time, the sulfuric acid aqueous solution can be used that the concentration is 4.9 to 98%.

When the organic acids are removed by leaching, a nano-sized channel is formed inside, allowing ions to move through the channel.

When aconic acid or itaconic acid is used as the organic acid, discoloration occurs due to carbon double bonds, but hydrophilicity can be imparted to the surface of the PVDF. This is also because these organic acids all have a hydrophilic —COOH group. Even when citric acid is used, some of it is converted to aconic acid or itaconic acid during heat forming to produce a hydrophilic surface.

In other words, the molded sample should give hydrophilicity to the surface and the remaining organic acid must be removed by leaching. When the reaction is carried out using a solvent, the color of the sample is changed from dark brown to light brown or light yellow if there is no change. Wash it.

The microporous membrane with the organic acid removed is finally immersed in an acidic electrolyte solution, leaving the acid in the pores. In this way, the acid remaining in the pores increases the amount of hydrogen ions, that is, increases the conductivity. In fact, when treated in sulfuric acid solution was 10 ^-3 Scm ^-1 when measuring the electrical conductivity, it was confirmed that the change to 10 ^-6 Scm ^-1 when the electrical conductivity was measured again after maintaining in boiling water for 1 hour.

As a result, the hydrogen ion conductivity of the PVDF resin membrane prepared according to the present invention is a result obtained by the hydrogen ions present in the micropores and the hydrophilicity on the surface. The transparency of the solution or shaped body was maintained when visually observed at all stages of manufacture.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, these Examples are only illustrative of the present invention, and the scope of the present invention is not limited to these.

The PVDF resin used in the following examples has a solid content of 45%, and the solid content has a molecular weight of 500,000 to 600,000. PVDF in pellet or powder form is dissolved in a solvent to form a transparent, high viscosity liquid. The available solvents include isophorone, DMF, DMS, DMAc and N-methyl-2-pyrrolidone. For convenience, in this experiment, dissolution was carried out using DMF, and a part of isophorone or N-methyl-2-pyrrolidone was added thereto to adjust the working temperature of the resin. DMF is a strong solvent, but the boiling point is 152.8 ℃, there is a problem that all is removed when the temperature is raised above. The boiling point of N-methyl-2-pyrrolidone is 202 ° C. and isophorone is 215.2 ° C., which is quite high, so that the resin can be kept flexible and eutectic with organic acids during the operation. In other words, the solvent used in this experiment was a mixture of the two, and some solvents were removed again to form a 45% solution. These organic acids are also very homogeneously dissolved with isophorone and other solvents (DMF, DMS, DMAc).

Electrical conductivity was measured using an H-cell in 1N sulfuric acid solution. The H-cells are made of glass, with the membrane sandwiched in the middle, filled with 1N sulfuric acid solution on both sides, and a platinum electrode placed in each compartment close to the membrane to measure electrical conductivity. The potential difference across the membrane is measured using two platinum wires that are in contact with both sides of the membrane, with a constant current flowing through the platinum electrodes at both ends.

Example 1

1.0 g of anhydrous citric acid (15 parts of citric acid: 100 parts of PVDF) was dissolved in 20 ml of DMF, and then 15 g of 45% PVDF resin was added. The mixture was heated to 100 ° C. or higher while stirring to completely dissolve the resin while removing a part of the solvent, thereby obtaining a clear solution. At or below 100 ° C., citric acid is maintained at 100 ° C. or higher because it absorbs moisture in the air and precipitates out of the sample as monohydrate. The solution was poured into a glass petri dish and waited for a bubble-free flat solution to be dried and molded at 120 ° C. for 6 hours in an oven. The sample was light brown and was taken out in boiling water and kept for 1 hour. The color turned pale yellow. The prepared PVDF membrane had a thickness of 102 μm and an electrical conductivity of 7.27 × 10 ⁻⁶ Scm ⁻¹ .

Example 2

2.0 g of anhydrous citric acid (30 parts of citric acid: 100 parts of PVDF) was dissolved in 40 ml of N-methyl-2-pyrrolidone and 15 g of 45% PVDF resin was added thereto. It heated to 190 degreeC and stirred, stirring. During heating, citric anhydride decomposes and turns into aconic acid and itaconic acid. When the acid and gel form a hard sample, thermoforming was performed at a pressure of 10 kg / cm 2 at about 170 ° C. The molded sample was kept in boiling water for 1 hour. The electrical conductivity of the prepared PVDF membrane was 5.2 × 10 ⁻⁶ Scm ⁻¹ .

Example 3

5.0 g of anhydrous citric acid (74 parts of citric acid: 100 parts of PVDF) was dissolved in 20 mL of DMF, and then 15 g of 45% PVDF resin was added. Upon stirring, the resin completely dissolved to obtain a clear solution. The solution was heated to remove the solvent to form a gel state. The gel was held at 120 ° C. and molded. The film obtained was 90 mu m thick. The film was placed in water and boiled to remove citric acid from the film, at which time the electrical conductivity was 3.78 × 10 ⁻⁴ Scm ⁻¹ . The film was placed in a 1 N (4.9%) sulfuric acid solution and maintained at room temperature for 20 hours. The measured electrical conductivity was 1.01 × 10 ⁻³ Scm ⁻¹ .

Example 4

5.0 g of anhydrous citric acid (74 parts of citric acid: 100 parts of PVDF) was dissolved in 20 ml of DMS, and then 15 g of 45% PVDF resin was added. Upon stirring, the resin completely dissolved to obtain a clear solution. The solution was heated to remove the solvent to form a gel state. The gel was held at 120 ° C. and molded. The obtained film was poured into water and boiled to remove citric acid from the film. The film with the citric acid leached out was placed in a 1 N sulfuric acid solution and maintained at room temperature for 20 hours, and the electrical conductivity measured was 1.04 × 10 ⁻³ S cm ⁻¹ .

Example 5

8.0 g of anhydrous citric acid (120 parts of citric acid: 100 parts of PVDF) was dissolved in 20 mL of DMF, and then 15 g of 45% PVDF resin was added. Upon stirring, the resin completely dissolved to obtain a clear solution. The solution was heated to remove the solvent to form a gel state. The gel was held at 120 ° C. and molded. The obtained film was poured into water and boiled to remove citric acid from the film. The film was placed in a 1 N sulfuric acid solution and maintained at room temperature for 20 hours, and the electrical conductivity measured was 2.4 × 10 ⁻³ Scm ⁻¹ . The films obtained were slightly yellow or brown and wet well with water. This is because citric acid loses water during heating and becomes aconic acid or itaconic acid, creating a polymerized hydrophilic surface on the PVDF surface.

Example 6

2 g of aconitic acid (30 parts of aconitic acid: 100 parts of PVDF) were dissolved in 20 mL of DMF, and then 15 g of 45% PVDF resin was added. It was confirmed that it became a transparent solution by stirring. The solution was poured into a petri dish and held at 180 ° C. in the oven for 1 hour. The solvent was completely evaporated off and a brown film was obtained. The film was soaked in 98% sulfuric acid for 2 hours and then washed to remove the remaining aconic acid. After removing the aconitic acid, the sample automatically contains sulfuric acid. The obtained PVDF membrane was a light brown transparent body having a thickness of 105 μm and had an electrical conductivity of 3.8 × 10 ⁻³ S cm ⁻¹ .

Example 7

2.1 g of itaconic acid (32 parts of itaconic acid: 100 parts of PVDF) was dissolved in 20 ml of DMF, followed by addition of 14.5 g of 45% PVDF resin. After stirring and heating, it was poured into a petri dish and cured at 160 ° C. After removing the sample, it was put in 98% sulfuric acid solution to remove excess itaconic acid. After itaconic acid is removed, sulfuric acid is automatically contained inside. The resulting PVDF membrane had a thickness of 145 μm and an electrical conductivity of 5.0 × 10 ⁻³ Scm ⁻¹ .

As described above, according to the method of the present invention, two characteristics required for the PVDF membrane to be used as a hydrogen ion conductor or a filtration membrane for water treatment can easily achieve ion conductivity and hydrophilicity. That is, according to the method of the present invention, the PVDF resin is dissolved in an appropriate solvent with a water-soluble organic acid such as citric acid, aconic acid, or itaconic acid, dried and molded, and then eluted and removed from the aqueous solution. As the organic acid polymerizes, a hydrophilic surface is formed, and the remaining organic acid escapes and the electrolyte is filled in the remaining micropores to have ion conductivity, and thus it can be used as a hydrogen ion conductor or a water treatment membrane.

Claims (6)

Adding an organic acid and a solvent capable of dissolving both to polyvinylidene fluorine resin to form a mixed solution; Evaporating and hot forming the solvent in the solution such that a portion of the organic acid is polymerized to form a hydrophilic surface; And By immersing the molded sample in an aqueous solution of water or acid, the remaining organic acid is removed from the sample to form micro pores so that the electrolyte solution is filled in the micro pores of the hydrogen ion conductive polyvinylidene fluorine resin membrane. Manufacturing method. The method of claim 1, wherein the organic acid is citric acid, aconic acid, itaconic acid, or a mixture thereof. The method according to claim 1 or 2, wherein the organic acid is mixed and added to the polyvinylidene fluorine resin at a ratio of 15 to 120 parts by weight based on 100 parts by weight of the solid content. The method of claim 1, wherein the solvent capable of dissolving both the polyvinylidene fluorine resin and the organic acid is dimethylformamide, dimethyl sulfoxide, dimethylacetamide, N-methyl-2-pyrrolidone, isophorone, or a mixture thereof. Method characterized in that. The method of claim 1, wherein the molded sample is immersed in boiling water or sulfuric acid aqueous solution to remove the remaining organic acid from the sample to form fine pores. The method of claim 5, wherein the aqueous sulfuric acid solution is characterized in that the concentration of 4.9 to 98%.